I recently raised some Monarchs from egg. I had several that did not hatch. They got dark, but nothing came out. And, some of the caterpillars leaked green and one turned dark and died. This does not bode well for the start of the season.
I always take the eggs, clip a small part of the leaf with the egg, put them on to a deli plastic lid, I use large 13 cup plastic glad containers, inside of that large container, I put down paper towel, dampen that, then put the plastic deli lid on top of the damp paper towel. This keeps the humidity up, but doesn't mold the leaf. I keep an eye on those for hatching caterpillars and move them to another container because the new hatchlings will crawl over and eat another egg.
Herndon, VA (USA)
Take care of the small things....